Method of controlling drum temperature transients

ABSTRACT

An evaporator system comprises an evaporator; a drum; and a pump that are in fluid communication with each other. The pump is operative to create a temporary pressure gradient during start-up of an evaporator system and transport a fluid from the evaporator to the drum prior to the fluid reaching its boiling point in the evaporator. Following the fluid reaching its boiling point in the evaporator, the fluid naturally circulates in the evaporator system.

TECHNICAL FIELD

Disclosed herein is a method of controlling drum temperature transientsin an evaporator system in a heat recovery steam generator. Morespecifically, disclosed herein is a method of using temporary forcedcirculation during startup to control drum temperature transients in aheat recovery steam generator.

BACKGROUND

Heat recovery steam generators generally comprise three majorcomponents: an evaporator, a superheater and an economizer The differentcomponents are put together to meet the operating requirements of theunit. Some heat recovery steam generators may not have a superheater ormay include additional components such as reheaters.

The FIG. 1 is a depiction of an exemplary prior art evaporator system100 of a heat recovery steam generator that comprises an evaporator 102and a steam drum 104. The steam drum 104 is in fluid communication withthe evaporator 102. In a natural circulation heat recovery steamgenerator, either no flow or minimal flow is established until boilingbegins in the evaporator 102. This generally results in a very rapidrise in the steam drum 104 temperature.

For example, for a cold start the water temperature inside the steamdrum 104 can rise from 15° C. to 100° C. in less than 10 minutes. Thisproduces a large thermal gradient and hence compressive stress in thesteam drum 104 wall. As the pressure in the steam drum 104 increases,the temperature gradient through the drum wall is reduced andconsequently the stress due to pressure becomes the dominant stress inthe drum. The stress due to pressure (with increased pressure in thesteam drum 104) is a tensile stress. The stress range for the drum isdetermined by the difference between the final tensile stress at fullload (pressure) and the initial compressive thermal stress. BoilerDesign Codes (such as ASME and EN) impose limits on the stress at designpressure. Some codes, such as for example EN12952-3, also include limitson the permissible stress range for a startup-shutdown cycle. Theselimits are intended to protect against fatigue damage and phenomena suchas cracking of the magnetite layer that forms on the surface of thesteel at operating temperature.

As the pressure in the steam drum 104 increases, the wall thickness ofthe steam drum 104 is also increased to ensure that the tensile stressin the drum shell at design conditions does not exceed allowable stresslimits specified in the design Codes. The thermal stress however,becomes greater as steam drum 104 wall thickness increases. The maximumpressure that a drum can be designed for is thus limited by the initialthermal transient.

It is also desirable to have as much operational flexibility as isdesirable for combined cycle power plants because these power plants areoften shut down and restarted as electrical power demand varies. Theaddition of renewable energy sources such as solar and wind increasesthe need to shut down and restart combined cycle power plants due to thevariation in power output from such renewable resources. Stresses in thedrum during these start ups due to thermal transients can also limit thetotal number of times the heat recovery steam generators can be shutdown and started over its operational life.

It is therefore desirable to reduce the temperature transient in thedrum. This will allow the use of drum type boilers at higher pressuresthan can be achieved with conventional natural circulation and/or allowgreater numbers of start up cycles.

SUMMARY

Disclosed herein is a method comprising creating a temporary pressuregradient during start-up of an evaporator system, where the evaporatorsystem comprises an evaporator; a drum; and a pump; where theevaporator, the drum and the pump are in fluid communication with eachother; transporting a fluid from the evaporator to the drum prior to thefluid reaching its boiling point in the evaporator; and circulating thefluid through the evaporator system via natural circulation after thefluid has reached its boiling point in the evaporator.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a prior art depiction of the evaporator system;

FIG. 2 is a depiction of an exemplary embodiment of the evaporatorsystem of the present invention; and

FIG. 3 is another depiction of an exemplary embodiment of the evaporatorsystem of the present invention.

DETAILED DESCRIPTION

The invention now will be described more fully hereinafter withreference to the accompanying drawings, in which various embodiments areshown. This invention may, however, be embodied in many different forms,and should not be construed as limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the scope of theinvention to those skilled in the art. Like reference numerals refer tolike elements throughout.

It will be understood that when an element is referred to as being “on”another element, it can be directly on the other element or interveningelements may be present therebetween. In contrast, when an element isreferred to as being “directly on” another element, there are nointervening elements present. As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, third etc.may be used herein to describe various elements, components, regions,layers and/or sections, these elements, components, regions, layersand/or sections should not be limited by these terms. These terms areonly used to distinguish one element, component, region, layer orsection from another element, component, region, layer or section. Thus,a first element, component, region, layer or section discussed belowcould be termed a second element, component, region, layer or sectionwithout departing from the teachings of the present invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willbe further understood that the terms “comprises” and/or “comprising,” or“includes” and/or “including” when used in this specification, specifythe presence of stated features, regions, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, regions, integers, steps,operations, elements, components, and/or groups thereof

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toanother element as illustrated in the Figures. It will be understoodthat relative terms are intended to encompass different orientations ofthe device in addition to the orientation depicted in the Figures. Forexample, if the device in one of the figures is turned over, elementsdescribed as being on the “lower” side of other elements would then beoriented on “upper” sides of the other elements. The exemplary term“lower,” can therefore, encompasses both an orientation of “lower” and“upper,” depending on the particular orientation of the figure.Similarly, if the device in one of the figures is turned over, elementsdescribed as “below” or “beneath” other elements would then be oriented“above” the other elements. The exemplary terms “below” or “beneath”can, therefore, encompass both an orientation of above and below.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to crosssectional illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

Disclosed herein is an evaporator system that comprises a pump forcirculating heated fluid from the evaporator to the steam drum. The pumpprovides circulation during start-up to initiate heating of the steamdrum, which reduces the rate of temperature change in the drum. Thisreduction in the rate of temperature change in the steam drum causesreduced thermal stresses in the drum. In an exemplary embodiment, thefluid is water.

The pump may be a centrifugal pump, a jet-pump pump, or the like, andits purpose is to provide a pressure gradient in the evaporator systemthat promotes fluid circulation from the evaporator to the steam drumbefore fluid (e.g., water) present in the evaporator begins to boil. Inone embodiment, the pump produces a lower pressure in the steam drum inrelation to the evaporator before fluid present in the evaporator beginsto boil. Upon generating a lower pressure in the steam drum, fluid fromthe evaporator is drawn into the steam drum causing the drum to heat upgradually. The gradual heating takes place until the fluid in theevaporator reaches the boiling point, at which point the pump may beshut off or isolated. After the pump is shut off, natural circulationpromotes circulation of the fluid in the evaporator system.

The pump therefore operates for a short period of time, until the steamdrum reaches the temperature of the boiling fluid. This allows for apump that is smaller in size than other comparative pumps that arenormally used. It also reduces stress in the wall of the steam drum.

With reference to the FIG. 2, an evaporator system 200 of the presentinvention comprises an evaporator 202, a steam drum 204 and a pump 206.The pump 206 is in fluid communication with the steam drum 204 and theevaporator 202. In one embodiment, the pump 206 lies downstream of thesteam drum 204. The steam drum lies downstream of the evaporator 202.

Disposed across the inlet and outlet to the pump 206 is a one-way checkvalve 208. The check valve 208 permits only fluid flow from the steamdrum 204 downstream to the evaporator 202 via the pump 206. The checkvalve further permits only fluid flow from the evaporator 202 downstreamto the steam drum 204. The pump 206 has a first valve 210 and a secondvalve 212 disposed upstream and downstream of it respectively. The firstvalve 210 and the second valve 212 can isolate the pump 206 from theevaporator system 200 when desired. The first valve 210 and the secondvalve 212 can be electrically, pneumatically or manually activated.

In one embodiment, in one method of operation of the evaporator system200, the pump 206 is used to circulate fluid from the evaporator 202 tothe steam drum 204 during start up of the heat recovery steam generatorto eliminate the rapid drum temperature rise that would normally occurin a natural circulation heat recovery steam generator. Once the steamdrum 204 temperature reaches a predetermined value, the pump 206 isisolated and the evaporator 202 runs under natural circulation. As thepump 206 can be isolated after start-up, it does not have to be sizedfor full flow load, pressure and temperature. This reduces the cost ofthe pump 206 when compared with comparative pumps that are used for fulltime circulation.

In another embodiment, depicted in the FIG. 3, the evaporator system 200comprises a jet-pump 306 (eductor) that creates a pressure gradient inthe evaporator system that promotes fluid circulation from theevaporator 202 to the steam drum 204 before fluid (e.g., water) presentin the evaporator 202 begins to boil. In one embodiment, the jet-pump306 produces a lower pressure in the steam drum in relation to theevaporator before the fluid present in the evaporator begins to boil.

The jet-pump 306 creates low pressure in a downcomer 308 that is influid communication with the steam drum 204 as a result of which fluidis drawn into the steam drum 204 from the evaporator 202. High velocityfluid flow in the narrow downcomer 308 induces a low pressure in thedowncomer 308 relative to the steam drum 204, which in turn causes flowin the downcomer 308. When the low pressure is created in the downcomer308, the steam drum 204 is at a lower pressure than the evaporator,which causes the fluid to flow from the evaporator 202 to the steam drum204. In one embodiment, low pressure created in the downcomer 308 by theoperation of the jet-pump 306 drives the circulation of fluid from theevaporator 202 to the steam drum 204.

The jet-pump 306 is in fluid communication with a first valve 310 and asecond valve 312. The first valve 310 is used to control the flow offeed water into the steam drum 204, while the second valve 312 is usedto isolate the jet-pump 306 from the downscomer.

The jet-pump 306 of the FIG. 3 functions in a manner similar to the pump206 of the FIG. 2 in that it permits a temporary fluid flow from theevaporator 202 to the steam drum 204 before the fluid present in theevaporator 202 begins to boil.

As noted above, the use of a pump for temporary circulation of fluid tothe steam drum has a number of advantages. These include using a pumpthat is smaller in size than other comparative pumps that are normallyused. It also reduces stress in the wall of the steam drum and permitsthe use of steam drums with larger wall thickness than those that arecurrently used in evaporator systems that do not employ temporarycirculation. This in turn allows operation of the steam drum at higherpressures or greater numbers of stop-start cycles.

While the invention has been described with reference to variousexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof Therefore, it is intended that the invention notbe limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method comprising: creating a temporary pressure gradient duringstart-up of an evaporator system, where the evaporator system comprises:an evaporator; a drum; and a pump; where the evaporator, the drum andthe pump are in fluid communication with each other; transporting afluid from the evaporator to the drum prior to the fluid reaching itsboiling point in the evaporator; and circulating the fluid through theevaporator system via natural circulation after the fluid has reachedits boiling point in the evaporator.
 2. The method of claim 1, where thepump is a centrifugal pump or a jet pump.
 3. The method of claim 1,where the fluid is water.
 4. The method of claim 1, where the fluid issteam.
 5. The method of claim 1, where the evaporator system furthercomprises valves for isolating the pump from the evaporator system. 6.The method of claim 1, where the evaporator system further comprises adowncomer; the pressure gradient being created in the downcomer from aregion of lower pressure in the steam drum to a region of higherpressure in the evaporator.
 7. The method of claim 1, where the pump isdownstream of the steam drum and upstream of the evaporator.
 8. Themethod of claim 2, where the jet pump is downstream of the steam drumand in fluid communication with a downcomer that is in fluidcommunication with the steam drum.